Ink jet printing is a non-impact method for producing images by the deposition of ink droplets on a substrate (paper, transparent film, fabric, etc.) in response to digital signals. Ink jet printers have found broad applications across markets ranging from industrial labeling to short-run printing to desktop document and pictorial imaging.
In ink jet recording processes, it is necessary that the inks being used meet various performance requirements. Such performance requirements are generally more stringent than those for other liquid ink applications, such as for writing instruments (e.g., a fountain pen, felt pen, etc.). In particular, the following conditions are generally required for inks utilized in ink jet printing processes:
(1) The ink should possess physical properties such as viscosity, surface tension, and electric conductivity matching the discharging conditions of the printing apparatus, such as the driving voltage and driving frequency of a piezoelectric electric oscillator, the form and material of printhead orifices, and the diameter of the orifices;
(2) The ink should be capable of being stored for a long period of time without causing clogging of printhead orifices during use;
(3) The ink should be quickly fixable onto recording media, such as paper, film, etc., such that the outlines of the resulting ink dots are smooth and there is minimal blotting of the dotted ink;
(4) The printed image should be of high quality, such as having a clear color tone, high density, high gloss, and high color gamut;
(5) The printed image should exhibit excellent waterfastness (water resistance) and lightfastness (light resistance);
(6) The printed (ink) images should have good adhesion to the surface of image receiving elements and should be durable and highly resistant to physical and mechanical scratches or damages;
(7) The ink should not chemically attack, corrode, or erode surrounding materials such as the ink storage container, printhead components, orifices, etc;
(8) The ink should not have an unpleasant odor and should not be toxic or inflammable; and
(9) The ink should exhibit low foaming and high pH stability characteristics.
The inks used in various ink jet printers can be classified as either dye-based or pigment-based. Pigment-based inks have some deficiencies. For example, pigment-based inks interact differently with specially coated papers and films such as transparent films used for overhead projection and glossy papers or opaque white films used for high quality graphics and pictorial output. This results in images that have poor dry and wet adhesion properties and that can be easily smudged.
Another deficiency from which pigmented inks suffer is their poor storage stability due to the presence of a water-miscible organic solvent. Water-miscible organic solvents are used to adjust ink rheology, to maximize ink firability and re-runability. These solvents prevent ink from drying in a printing head and lower ink surface tension to minimize the effect of air entrapment in an ink formulation. Such air entrapment generates air bubbles which can seriously affect the head performance. Unfortunately, these water-miscible organic solvents can also have a negative effect on the colloidal stability of pigment particles in an ink formulation.
A dye is a colorant which is molecularly dispersed or solvated by a carrier medium. The carrier medium can be a liquid or a solid at room temperature. Known ink jet inks generally comprise a water soluble dye which is soluble in an ink vehicle such as water or a mixture comprising water and a known water soluble or water miscible organic solvent. Inks comprising these soluble dyes may exhibit many problems, such as low optical densities on plain paper, poor water-fastness, poor lightfastness, clogging of the jetting channels as a result of solvent evaporation, and changes in the solubility of the dyes, dye crystallization, and ink bleeding. There is a need to develop a water resistant dye-based ink and to improve the existing inks. More specifically, there is a need to develop inks that provide high quality prints on a wide variety of recording media, including plain paper. Particularly, there is a great need to develop dye-based inks which have high optical densities on receivers and also superior lightfastness and colorfastness. These and other needs may be achievable in embodiments of the present invention.
Oil soluble (water insoluble) dyes have been studied to achieve these requirements. Some inks use organic solvents, but such inks have environmental considerations. Water-based oil soluble dye inks have the advantages of low pollution, low price, and good quality. It is difficult, however, to form a stable dispersion of oil soluble dye in water media. WO 98/14524 and U.S. application Ser. No. 09/510,879 disclose oil soluble colorants which have been dissolved in organic solvents and added to aqueous polymer latexes slowly under agitation. After mixing, the organic solvent is evaporated and the colorant is loaded to the polymer particles. The mixture is used to formulate ink jet inks with the addition of water, humectants, and some other additives. U.S. Pat. No. 5,958,998 discloses an ink composition containing an oil soluble colorant. The composition is made by flushing pigment into a sulfonated polyester resin having a degree of sulfonation of about 2.5 to about 20 mol percent, dispersing the pigmented polyester resin in water at a temperature of from about 40° C.-95° C., and thereafter separating and mixing. The dispersing is done by a shearing device operating at speeds of 100 to 5000 revolutions which yields stable pigmented submicron sized particles of from about 5 to about 150 nanometers. However, the amount of colorant that may be loaded in the polymer is limited.
JP 00297126A discloses a process for making an ink composition wherein polymer-colorant particles are prepared using a batch emulsion polymerization process employing colorant, monomers, and an initiator. This process, however, involves more micelle particle nucleation and polymer particles are produced. When formulated into an ink jet ink, the presence of free polymers (both polymer particles and water-soluble polymers) can significantly increase ink viscosity, decrease ink storage stability, cause premature printing head failure, and generate image defects.
There is still needed a colorant particle dispersion that has improved compatibility with water-soluble organic solvents and improved stability in an ink jet composition. There is further a need for an ink composition which, when printed, provides images on the surface of an ink jet recording element having improved image quality, improved physical durability such as scratch and smudging resistance, and improved image stability such as colorfastness and waterfastness.